Charging 12V gell cell in ones car

[Andrew Leigh]

Hi,

my friend wants to be able to charge his 12V 7.2Ah gelled cell whilst driving in his car. This is in emergencies only and will not be the norm. I found the below mentioned circuit and was hoping it will work. I am a little worried that the voltage output from the alternator will not be high enough.

If the circuit will not work could someone head me in the right direction. Could one not simply use a current limiting resistor and a timer?

Cheers
Andrew

 

[MikeMl]

No, this supply will not work; there is not enough head room.

The rapid charge voltage for an SLA is 14.7V. The long term float voltage for an SLA is 13.7V. The alternator in the car (above engine idle RPM) puts out 14.2 to 14.5V.
Simply paralleling the SLA across the car's battery will do as long as the SLA is not left connected more than several hours per charge cycle.

 

[Andrew Leigh]

Thanks Mike,

would I need to limit the current to the SLA?

Cheers

 

[MikeMl]

How badly discharged is the SLA prior to connecting it? If it has been discharged below 12.0V, then some sort of current limiting is required. I would use a 1Ω or 2Ω, several Watt resistor in the positive lead...

 

[Andrew Leigh]

Thanks Mike,

was thinking of building him a low voltage cutout so he does not discharge the battery too deeply. But will insert a resistor for safety sake.

Andrew

 

[k7elp60]

I would be somewhat cautious of charging a 7.2AH Gel-Cel from a automobile system. And especially if the connection to the Gel-Cel battery is via a cigrette lighter socket. Most manufacturers recommend that the charging current at the 14.4 to 14.7 volt range be limited to 20% of the capacity. In this case 7.2 X .2 or 1.44 amps. If the battery is connected to cigarette lighter plug and is left connected when the engine is turned off and sometime later the engine is started there my be excessive current out of the Gel-Cel that could result in a fire. When I use Gel-Cel batteries I always install a fuse as the first connection between the positive lead and any load. I have the fuse a fast blow and about 50% higher than my maximum normal load. The average Gel-Cel battery can produce about 10X or more of it's ampere hour capacity in a short period with a short or near short across it's terminals. I have seen load wires turn instantly orange and fill a room with smoke with a short or a near short.
What I have done in the past to solve the charging current problem and prevent the Gel-Cel from discharging when starting a car is forward voltage match some schottky 3A diodes like 1N5820's and put a small number in parallel, along with the fuse connected to the cigarette lighter socket or accessory socket as needed. The diodes prevent the battery discharging into the cars electrical system and also reduce the charge voltage so the battery's never overcharg.

 

[Hero999]

I don't think it needs to be very complicated, even if you want to implement a full battery protection system.

A 1R resistor should limit the current to a safe level and if you're worried about it being plugged in for too long then you could use a CD4060 oscillator/counter IC as a timer to cut off the power, if it's left connected for too long.

If you're worried about over-discharge a comparator and voltage reference could cut the power when the voltage drops below 10.5V, or higher. If you're clever then you could design it to reset the timer when the voltage drops below 12.5V (or even higher) so it doesn't have to go through a full cycle. You'll need some time delay for the comparators as you don't want current surges to trip the protection but that's as simply as adding a capacitor to a potential divider to form an RC circuit. A PTC resistor (polyfuse) could be used as over current protection rather than a fuse which needs replacing every time it's tripped.